In an effort to get to the bottom of this, did you try without using network-manager ?
Because it is difficult to tell if the problem is with openvpn or network-manager.

In this case, openvpn is started outside of NetworkManager. And a server configuration is attempted to be started. So "not using network-manager" would mean uninstall it; which I strongly discourage. That can cause lots of other issues on top of this.

In an effort to get to the bottom of this, did you try without using network-manager ?
Because it is difficult to tell if the problem is with openvpn or network-manager.

In this case, openvpn is started outside of NetworkManager. And a server configuration is attempted to be started. So "not using network-manager" would mean uninstall it; which I strongly discourage. That can cause lots of other issues on top of this.

@dazo , why would be an issue not using network-manager to starting openvpn and also why would that mean to uninstall network-manager ?
the only case where i would see network-manager interfere is on a workstation with a gui, and even there only if you use the network-manager openvpn plugin.

anyhow , i just installed openvpn on a debian 9 system and don't have any problems , but i have to say that compared to RHEL based family of linux distros , on debian... systemd is a bad joke , lot's of service units actually just run an old school init script and that's exactly the case for openvpn

so just start openvpn like you would on an older system which does not use systemd

@TiTex ... I understand that systemd on Debian can feel like a bad joke. I don't think it's that bad, but what is bad is all the wrappers put around systemd to make it behave like a non-systemd system. Like you point out with using /etc/init.d/openvpn directly ... but I don't recall which way it wraps; if systemd is used as a wrapper around init.d scripts, well, that is going to cause a nastier user experience.

Which is why we managed to convince the Debian package maintainer to ship our openvpn-{client,server}@.service files ... but I'm not sure it has been updated with the changes we added in later 2.4.1+ releases; I've seen some traces of them being the same as in 2.4.0, which carries some bugs. So if the package maintainer could pick up our latest upstream versions and ship them, I believe Debian users could experience a much better systemd integration. But the old way of interacting with initd scripts needs to stop, it will give a half-baked experience.

In regards to uninstalling network-manager ... I don't know how tightly that is integrated into the distribution in Debian. But if you try to do such a stunt in RHEL/Fedora, you end up with a system which won't configure network interfaces well during boot. You can make it work by adding some tweaks in the various systemd network configuration unit files and so on. NetworkManager actually can do good things on servers as well as workstations and in particular laptops where networks are even less static. But in today's virtualized world, networks are not as static as they used to be with essentially just bare-metals in the data centres.

Now another "nmcli dev status" should show that Network Manager is no longer in control of your Network interfaces.

Also Note: I had to append the following line to /etc/group:

nobody:x:65534:nobody

Why? Because in Debian 9, you get a "nobody" user by default, in /etc/passwd, but no "nobody" group in /etc/group. Once the "nobody" group was created thusly, then and only then I could use "user nobody" and "group nobody" in server.conf.

I would rather recommend you to not disable NetworkManager on the system as a whole. Rather tell NetworkManager to not touch certain devices; like tun/tap devices. In /etc/NetworkManager/NetworkManager.conf rather add:

Really. Stop believing NetworkManager is evil. It is trying to do all it can to make networking work nicely in todays more dynamic network environments. Killing it will give you other challenges when the environment around your box changes. Telling it "I know what I'm doing with these devices, you can relax" will generally give you a better experience in the end.